Concurrent modification in event producer

OneSignalSDK/onesignal/core/src/main/java/com/onesignal/common/events/EventProducer.kt

@@ -45,10 +45,9 @@ open class EventProducer<THandler> : IEventNotifier<THandler> {
      * @param callback The callback will be invoked for each subscribed handler, allowing you to call the handler.
      */
     fun fire(callback: (THandler) -> Unit) {
-        synchronized(subscribers) {
-            for (s in subscribers) {
-                callback(s)
-            }
+        val localList = subscribers.toList()
+        for (s in localList) {
+            callback(s)
         }
     }
 
@@ -61,7 +60,8 @@ open class EventProducer<THandler> : IEventNotifier<THandler> {
      */
     fun fireOnMain(callback: (THandler) -> Unit) {
         suspendifyOnMain {
-            for (s in subscribers) {
+            val localList = subscribers.toList()
+            for (s in localList) {
                 callback(s)
             }
         }
@@ -74,7 +74,8 @@ open class EventProducer<THandler> : IEventNotifier<THandler> {
      * @param callback The callback will be invoked for each subscribed handler, allowing you to call the handler.
      */
     suspend fun suspendingFire(callback: suspend (THandler) -> Unit) {
-        for (s in subscribers) {
+        val localList = subscribers.toList()
+        for (s in localList) {
             callback(s)
         }
     }
@@ -87,7 +88,8 @@ open class EventProducer<THandler> : IEventNotifier<THandler> {
      */
     suspend fun suspendingFireOnMain(callback: suspend (THandler) -> Unit) {
         withContext(Dispatchers.Main) {
-            for (s in subscribers) {
+            val localList = subscribers.toList()
+            for (s in localList) {
                 callback(s)
             }
         }

OneSignalSDK/onesignal/core/src/test/java/com/onesignal/common/ModelingTests.kt

@@ -9,6 +9,7 @@ import com.onesignal.mocks.MockPreferencesService
 import com.onesignal.user.internal.subscriptions.SubscriptionModel
 import com.onesignal.user.internal.subscriptions.SubscriptionModelStore
 import io.kotest.core.spec.style.FunSpec
+import io.kotest.matchers.shouldBe
 import io.kotest.runner.junit4.KotestTestRunner
 import junit.framework.TestCase
 import org.junit.runner.RunWith
@@ -70,13 +71,11 @@ class ModelingTests : FunSpec({
         val t1 =
             Thread {
                 // acquire "ModelStore.models", then trigger the onChanged event
-                System.out.println("1")
                 modelStore.add(newSubscriptionModel)
             }
 
         val t2 =
             Thread {
-                System.out.println("2")
                 // acquire "model.data", then wait for "ModelStore.models"
                 newSubscriptionModel.toJSON()
             }
@@ -116,4 +115,29 @@ class ModelingTests : FunSpec({
         // verify if the thread has been successfully terminated
         TestCase.assertEquals(Thread.State.TERMINATED, t2.state)
     }
+
+    test("Unsubscribing handler in change event may cause the concurrent modification exception") {
+        // Given an arbitrary model
+        val modelStore = MockHelper.configModelStore()
+        val model = modelStore.model
+
+        // subscribe to a change handler
+        model.subscribe(
+            object : IModelChangedHandler {
+                override fun onChanged(
+                    args: ModelChangedArgs,
+                    tag: String,
+                ) {
+                    // remove from "subscribers" while "subscribers" is being accessed
+                    model.unsubscribe(this)
+                }
+            },
+        )
+
+        // this will trigger EventProducer.fire and loop through the list "subscribers"
+        model.setOptAnyProperty("key1", "value1")
+
+        // ensure no concurrent modification exception is thrown and "subcribers" is clear
+        model.hasSubscribers shouldBe false
+    }
 })